Laminators are constantly being challenged to reduce or eliminate visual defects in laminated glass structures. Some defects can be directly attributed to glass quality but many are considered to be associated with the laminating process and more specifically with the commonly used polyvinylbutyral (PVB) interlayer. Defects can look like bubbles or pockets of air with elongated worm-like or dendritic shapes. Elongated worm-like and dendritic defects are often referred to as delamination. Some defects are visible immediately after autoclaving, but others develop hours or days after lamination. Laminators who use vacuum for de-airing tend to experience higher defect rates in warm weather.
Traditionally, delamination is viewed as the result of adhesive bond failure between the glass and the PVB interlayer. That is, the adhesive forces cannot withstand the stresses that are due to mismatches in the glass as well as gaps and pinches. A typical explanation for defects that are near a laminate""s edge, is that the PVB absorbs moisture from the environment, which lowers the adhesion level leading to defect formation. Therefore, it is rationalized that during warm and humid seasons, moisture is absorbed at a higher rate, and hence causes more defects.
In the laminating industry there is general agreement that gaps and pinches do lead to defects. In fact gaps of approximately 0.1 mm in height over a distance of 5 cm are suspects for causing defects. The load required to achieve a 0.1 mm gap or pinch can be calculated from mechanical considerations, and it is a surprisingly low, 1.0 N/cm for 2.1-mm thick glass. For this reason, adhesive forces cannot explain the formation of most defects.
If the adhesion level is primarily responsible for defects, then higher adhesion levels should be able to overcome more stress, and hence, would accommodate larger gaps and pinches without causing a defect. However, our findings have shown this not to be true.
With respect to moisture absorption, the adhesive interlayer absorbs moisture from the environment until equilibrium is reached. The equilibrium level depends on the relative humidity and may differ for different interlayers. The mechanism for moisture absorption is diffusion, which means that the concentration of the diffusant is highest at the phase boundary (i.e., at the laminate""s edge). A typical moisture profile of a PVB laminate exposed to 95% relative humidity at 40xc2x0 C. for one week shows that only interlayer within 3-4 mm from the edge has moisture higher than 1.5%, and the moisture level hardly changes about 8 mm in from the edge. Most of the observed defects occur about 3-12 mm away from the edge and some extend slightly farther inwards. Very few defects are open to the edge where the moisture level is highest and where one would expect to have the lowest level of adhesion.
It is possible to adjust the adhesivity of the PVB interlayer so that even when laminated at high moisture, the final adhesion is suitable for use in automobile windshields. However, laminates made this way would fail if they are installed into automobiles which are driven in or exposed to high ambient temperatures. Bubbles form readily at temperatures less than 100xc2x0 C. in laminates where the PVB interlayer has been equilibrated prior to lamination to a relative humidity higher than 50%. These laminates most likely would not pass the bake test or the boil test required by national and international standards (e.g., ANSI Z26, JIS R-3212, EC R-43, and others).
Another reason moisture intrusion does not explain many of the defects is that even in the absence of high moisture, adhesion at 30xc2x0 C. is only a fraction of what it is at room temperature. Increasing the adhesion between the glass and the adhesive interlayer at room temperature, therefore, would not help to eliminate defects which tend to occur at higher temperatures. Further, correlation between data from tests run at temperatures well below room temperature, such as the pummel test, and delaminations is at least questionable.
We have found that the presence of air plays a most significant role in defect formation in laminated glass. De-airing and edge seal must be as complete as possible in pre-pressed laminates before autoclaving in order to avoid defects. However, optimizing de-airing alone does not appear to solve the delamination problem completely.
The typical approach in attempting to solve delamination problems has been to include various additives in the adhesive sheet to increase the strength of the adhesive bond between the sheet and the glass plate. While such approaches have been successful in changing the adhesive level, and to some extent reducing delaminations, increasing adhesion upsets the delicate balance of properties which make laminated products so desirable in automotive and other fenestration applications. It is well known that an adhesion level that is too high can render the laminate monolithic and unable to absorb an impact, or if the adhesion is too low, glass shards fly from the structure on impact. In each instance, changing the adhesion level renders the laminate unacceptable.
It is therefore the object of this invention to provide a laminar structure which is free from objectionable worm-like, dendritic, delamination by imparting delamination resistance without adversely changing the adhesion level or other important properties of the laminar structure.
In accordance with this invention there is provided a glass/adhesive sheet laminar structure comprising at least one layer of glass and a sheet of plasticized PVB, said PVB having blended therein an adhesion control agent to provide a preselected level of adhesion between said layer of glass and said sheet of PVB which is suitable for use as automobile windshields, side windows and body glass, and incorporating a surface energy modifying agent in the bulk of the polymer in an amount so that said sheet of polyvinylbutyral interlayer has a total surface energy of less than about 52 dynes/cm.
It has now been found that dendritic or worm-like delaminations in PVB laminated structures result from air being dispersed, entrapped or dissolved in the PVB interlayer during autoclaving. The dissolved air when it exceeds its equilibrium solubility in PVB comes out of solution and causes defects to form. These delaminations first appear as tiny bubbles which grow or coalesce to form larger bubbles and eventually worm-like or dendritic delamination. The formation and stability of air bubbles in PVB, like other systems containing two immiscible phases, depend on thermodynamic conditions in which the primary driving force is to reduce the total interfacial area per unit volume between the two phases. That is why small bubbles join to form a single one of greater volume but smaller total interfacial area. The ability of an additive to keep the air dispersed depends primarily on its effectiveness at reducing the surface tension of PVB.
In accordance with this invention delamination is eliminated or substantially reduced by keeping the air dispersed and preventing microbubbles from coalescing and growing into delaminations. This is accomplished by controlling the surface energy in the bulk of the PVB sheeting. Generally, the surface energy should be less than about 52 dynes/cm. Surface energy in the range from about 35 to 50 dynes/cm is efficacious in stabilizing the undissolved air and volatiles without substantially changing the adhesion level between the glass and the PVB interlayer or the balance of properties of the PVB interlayer such as compliance, stiffness, energy-absorbing characteristics so that the laminate may be used in automotive windshield and other automotive applications.